In situ retorting of oil shale



Dec. 14, 1965 c. o. BAKER 3,223,153

IN SITU RETORTING OF OIL SHALE Filed Dec. 10, 1962 2 Sheets-Sheet 1 OIL+ GAS AIR FUEL FIG. 2

CHARLES OVID. BAKER INVENTOR.

BY W

ATTORNEY Dec. 14, 1965 c. o. BAKER 3,223,158

IN SITU RETORTING OF OIL SHALE Filed Dec. 10, 1962 2 Sheets-Sheet 2 FIG4 33 37 CHARLES OVID BAKER INVENTOR ATTORNEY United States Patent 3,223,158 IN SITU RETORTING OF OIL SHALE Charles Ovid Baker, Garland, Tex., assignor to Socony Mobil Oil Company, Inc., a corporation of New York Filed Dec. 10, 1962, Ser. No. 243,301 2 Claims. (Cl. 166-11) This invention relates to a process for the recovery of hydrocarbons from oil shale. More particularly, the present invention is directed to a process for in situ retorting and recovering of hydrocarbons from massive underground formations of oil shale.

Vast deposits of oil shale are located within the United States in underground formations. The oil shale in such deposits contains hydrocarbons in the form of kerogen. These hydrocarbons cannot be recovered by conventional petroleum producing methods. Shale containing kerogen must be subjected to retorting or destructive distillation to provide shale oil. The shale oil is a hydrocarbon product much like crude petroleum and includes liquid and gaseous materials which may processed to provide fuels and other useful products.

Much of the vast deposits of oil shale are in massive formations. The tremendous cliifs of oil shale exposed in the Green River Basin of Colorado and Wyoming provide visual evidence of the great expanse and thickness of these oil-containing formations. The oil shale generally is covered with an overburden of significant extent which may be 1000 to 2000 feet in thickness. Further, the oil shale in some locations may extend to a depth 4000 to 5000 feet below the overburden. In such massive formations, the shale oil recovered from a block of oil shale having a horizontal surface of even one acre and extending the entire depth of the oil shale contains a tremendous amount of valuable hydrocarbons.

Up to the present time no known process for recovering the shale oil from such massive oil shale formations has produced shale oil that from an economic standpoint could be reasonably compared to petroleum crude oil.

The known processes for producing shale oil employing conventional mining methods for removing the oil shale from its natural position, transporting the oil shale to the surface, and there retorting it to produce shale oil are not feasible from an economic standpoint. The known processes for producing shale oil by in situ retorting either fail to be usable at the great depths encountered in massive oil shale or they can recover only small amounts of shale oil so that economic considerations prohibit their' commercial use.

The problem which the known in situ oil shale recovering processes have failed to solve at great depths in massive oil shale is twofold. First, the oil shale has very low natural permeability and as a result, must be extensively fractured or broken into a rubble of great and uniform permeability so that in situ retorting of the shale rubble will permit uniform heat permeation in order to release substantially all the available shale oil. Second, the shale oil rubble must be contained in a substantially impervious natural environment as a retorting container to insure that substantially all of the shale oil produced by retorting can be recovered. The above problem becomes especially acute when attempts are made to in situ process oil shale more than 1000 feet below the surface of the earth where conventional mining and tunneling processes are an economic impossibility.

The present invention is directed to an economically feasible method for the in situ retorting and recovering of hydrocarbons from the massive underground formations of oil shale that have heretofore defied economic and practical recovery by known processes.

It istherefore an object of the present invention to Patented Dec. 14, 1965 provide a process for in situ retorting and recovering of shale oil from massive oil shale formations with greater operative facility, with maximum recovery of hydrocarbons, and at lower costs than in known processes.

Another object is to provide a process for in situ retorting and recovering of shale oil from great depths in oil shale by providiing an area of oil shale rubble having great and uniform permeability so that the rubble is readily permeated with heat sufiicient to drive substantial quantities of all available hydrocarbons from the oil shale.

Another object is to provide a process for in situ retorting and recovering of hydrocarbons from horizons in massive oil shale at depths where the high temperatures resulting from the geothermal gradient of the earth prevent the use of conventional mining techniques.

Another object is to provide a process for in situ retorting and recovering of hydrocarbons from massive oil shale utilizing conventional oil well directional drilling procedures to provide access to even the lowermost horizons in the oil shale with a minimum number of surfaceexposed boreholes.

Another object is to provide a process for in situ retorting and recovering of hydrocarbons from massive oil shale wherein the heating to drive the hydrocarbons from the shale may be initiated at one extreme horizontal level of the shale and progress vertically through the entire shale formation while the surrounding portions of the earths formations remain intact to provide a fluid-tight retorting container.

Another object is to provide a process for in situ retorting and recovering of hydrocarbons from massive oil shale from isolated portions of the formation and also for an expanding pattern of in situ retorting and recovering of hydrocarbons from the entire extent of the shale formation.

These and other objects will be more apparent when considered in conjunction with the following specification, the attached drawings, and the appended claims.

In the drawings are shown illustrative embodiments of the present invention wherein:

FIGURE 1 is an isometric view of a rectangular block of the earth showing an illustrative process embodiment with two boreholes for in situ retorting and recovering of shale oil from horizons at great depths within massive oil shale formations;

FIGURE 2 is the same as FIGURE 1 but an isometric view taken more vertically with a portion of the earth removed showing the intersecting course relationships of the two boreholes;

FIGURE 3 is a diagrammatic illustration of further illusrative embodiments of the present invention shown in FIGURES 1 and 2 with a plurality of boreholes, each having a multitude of deviated extensions for in situ retorting shale oil in several horizons at great depths within massive oil shale formations; and

FIGURE 4 is a vertical cross section of the structure of FIGURE 3 taken along line 44 with an area of oil shale rubble designated A surrounded by an impervious retorting container.

The objects are obtained by a process including the steps of providing one or more boreholes which extends from the earths surface downwardly into the oil shale with radially diverging borehole extensions obtained by directional drilling. The lower portions of the borehole extensions are drilled so as to deviate from the vertical toward the horizontal with one such extension having its lower portion disposed at the horizon in the oil shale from which sh-ale oil is to be recovered. The lower portions of such borehole extensions are spaced a distance permitting the oil shale adjacent to and intermediate the borehole extensions to be extensively fractured by conventional fracturing means from the borehole extensions.

fluids.

from adjacent such borehole extension progressively outwardly to a temperature producing and driving shale oil to the remaining borehole extensions and then recovering the shale oil from them.

Referring now to FIGURE 1, there is shown a rectangular block of the earth 11 in which the upper portion is a thin layer of overburden 12, and a lower portion is a formation of massive oil shale 13. The overburden 12, for example, may be about 1000 feet thick; and the oil shale 13, for example, may extend 5000 feet or more below the over-burden. The oil shale 13 contains striations 14 which are generally parallel to the bedding of the shale formation. The striations 14 are noncombustible rocks of very small thickness but of great strength and relatively low permeability. The striations 14 are believed to prevent obtaining satisfactory results from known in situ recovery processes using spaced vertical boreholes and passing in situ combustion heat waves therebetween to obtain the desired heating and increased permeability of the intervening oil shale. In some instances, the natural permeability may occur parallel to the striations, as may any artificially created increased permeability produced from vertical boreholes. Therefore, the vertical extent of the channels or flowways formed in the oil shale 13 by these processes can be very small. Thus, when in situ retorting is undertaken the heatfronts may travel only horizontally through the shale 13 because their vertical travel can be severely limited by striations 14. As will be seen from the discussion hereafter, the novel process of the present invention overcomes this difficulty created by striations 14 by producing a rubble area of high and uniform permeability in oil shale 13 containing multitudes of channels 16 or flowways along both horizontal and vertical planes throughout the rubble area.

More specifically, the process of this invention provides at least one borehole 17 extending from the earths surface downwardly into the oil shale 13. This borehole 17 may be formed by any suitable means, such as by conventional rotary drilling. From the borehole 17 there is provided by directional drilling a borehole extension 18 radially deviating from the vertical. The lower portion 19 of the borehole extension 18 diverges from the vertical toward the horizontal at the horizon in the shale 13 from which shale oil is to be recovered.

The art of directional drilling is well developed. Reference may be had to the Rotary Drilling Handbook, 6th edition, by J. E. Brantly, published by Palmer Publications, New York, New York, and specifically therein to Chapter XX, entitled Directional Drilling, at pages 380 through 423 for more detailed information. The diameter of the borehole and its directionally drilled extension is preferably of a large rather than a small diameter since it is easier to control the direction of the larger diameter boreholes. A vertical wellbore can be readily directionally drilled to provide deviating borehole extensions having a uniform deviation build-up of about 230 increase in drift for each 100 feet of hole directionally drilled. Simple calculations of the deviation build-up indicate that the terminal lower portions of the borehole extensions from a vertical borehole 1000 feet deep can deviate from the vertical at angles of about 80. Also, the borehole extensions can be directed with great accuracy on a given course towards a particular objective.

A second borehole 21 is provided which extends from the earths surface downwardly into the oil shale 13. A borehole extension 22 is provided by directional drilling from the borehole 21. The borehole extension 22 has a lower portion 23 diverging from the vertical toward the horizontal at a horizon different from the horizon containing the lower portion 19 of the borehole extension 18. Further, the borehole extension 22 is disposed in spaced relationship to the borehole extension 18. By in spaced relationship is meant that portions of the borehole extension 22 pass adjacent to the borehole extension 18 by a distance which will permit the intervening oil shale between the borehole extensions 18 and 22 to be extensively fractured by conventional fracturing means operated from such borehole extensions. This distance is dependent upon the etfectiveness of the fracturing means and usually may vary between about 5 to about feet or more. Some fracturing occurs to a smaller radial distance circumferentially along the entire length of each borehole extension.

Although the borehole extension 22 may be directionally drilled from the borehole 17, it is preferred that separate boreholes extend to the earths surface from the respective extensions in order that the necessary piping to supply a heat-creating gas to retort the shale and to remove the products of retorting be easily contained. As can be readily seen from FIGURE 1, the borehole extension 22 is spaced substantially vertically from the borehole extension 18 by the distance heretofore defined. This is a preferable arrangement that provides many advantages. One significant advantage is that the heat used in retorting the shale moves upwardly through the oil shale by natural drives if sufficient permeability is present. Therefore, the heating may be initiated adjacent the borehole extension 18 and readily passed upwardly toward the borehole extension 22 because the necessary permeability in the oil shale is provided. This, of course, produces greater thermal efliciency in the retorting of the oil shale for any given amount of a heat-creating gas than known processes which must propagate the heat fronts along a horizontal path. This vertical spaced arrangement of the lower portions 19 and 23 of borehole extensions 18 and 22 is of great advantage for the reason that fracturing the intervening oil shale increases to a great extent the amount of channels 1 6 or flow passageways which extend vertically. This function, in conjunction with the natural tendency of the oil shale to fracture horizontally, produces a rubble area in the shale of greatly increased permeability.

Further, it is preferred that the borehole extensions 18 and 22 be directionally drilled in a manner that their lower portions 19 and 23 diverge from the vertical towards the horizontal along intersecting courses. This can best be seen in FIGURE 2. By intersecting courses, it is meant courses passing across each other but not neces sarily meeting so that the borehole extensions 18 and 22 are in direct communication. With the lower portions of the borehole extensions on courses which intersect, the fracturing of the intervening oil shale produces vertical channels or flowways along many vertical planes which are substantially normal to the course the respectwo lower portions of the borehole extensions follow and also along a plurality of horizontal planes. By this means, channels 16 are formed which radiate in all directions to form a rubble area in the oil shale 13 of great and uniform permeability with a spherical configuration. The remainder of the oil shale surrounding such rubble area is not fractured so that it retains its inherent low permeability which, for practical purposes, is negligible. The reason for this is that the extent of fracturing is limited by the distance the borehole extensions lower portions are spaced apart. Thus, the oil shale rubble area is effectively contained in a substantially impervious environment as a retorting container so that all of the retorted shale oil can be recovered through the borehole extensions and the boreholes which extend to the surface of the earth.

After the step of directional drilling the borehole extensions 18 and 22 are completed, the oil shale adjacent to and intermediate the borehole extensions 18 and 22, and in particular, their lower portions 19 and 2.3 is, extensively fractured. The step of fracturing may be practiced by conventional fracturing means applied to the oil shale from the borehole extensions. Any suitable fracturing means can be used, such as explosive means or hydraulic means. Usually fracturing can be obtained for distances up to 100 feet or more, depending upon the fracturing power available from conventional fracturing means. However, it is desirable to install necessary fluid conduits in the borehole extensions for injecting the heating gases and removing the produced shale oil prior to fracturing. For example, the fracturing means are placed in the borehole extensions 18 and 22 and actuated to produce an area of oil shale rubble by creating a multitude of fractures, channels, or flowways, in the shale adjacent to and intermediate the borehole extensions. This oil shale rubble area is of a spherical configuration and has a uniform and great permeability throughout as a result of the fracturing forces acting along many horizontal and vertical planes about the borehole extensions. Stated in another manner, the defined fracturing occurs to produce the desired rubble area as a result of fracturing forces being applied radially from arcuate lines to the formation, i.e., from the borehole extensions. The oil shale beyond the defined distance from the borehole extensions remains intact and forms an impermeable container in which retorting can be effected. Thus, by using the directionally drilled borehole extensions in the manner described in the mentioned spaced relationship, an area of highly fractured shale is produced extending vertically through one or more horizons in the oil shale 13 from which hydrocarbon production is desired.

After fracturing, the oil shale in the rubble area is subjected to the step of heating to decompose the kerogen into shale oil. Preferably, the oil shale rubble is heated to temperatures such that the kerogen is above 650 F. As a result of the high degree of fracturing in the oil shale rubble area, and the surrounding impermeable unfractured oil shale, ready heating to a temperature to retort the shale oil from the rubble is obtained. Similarly, the shale oil is readily driven and educted to a common collection point by the relatively free passage of a heatcreating gas through the oil shale rubble under the ideal conditions of a spherical retorting container. If desired, several collection points disposed in the rubble may be used to insure recovery of all liquid and gaseous shale oil products.

The term heat-creating gas is used herein to include a gas selected from the group consisting of oxygen-containing gases singly or in various proportions and combination with combustible and noncombustible gases. The heat-creating gas may be applied to heat the oil shale rubble by direct or reverse in situ combustion as is ap parent to one skilled in the art.

More specifically, the area of oil shale ruble is heated by injecting a heat-creating gas through one of the borehole extensions into the rubble to heat same to the required temperature. The oil shale rubble is heated from adjacent such borehole extension progressively outwardly throughout the confines of the area of oil shale rubble. The heating is continued to provide a temperature sufficient to drive the hydrocarbons from the oil shale rubble to the other borehole extension where they can be recovered and educted to the surface of the earth for further processing into useful fuels and other products.

Preferably, the heat-creating gas is injected through the lower borehole extension 18 into the oil shale rubble and the shale oil, including whatever gases are produced, is recovered through the upper borehole extension 22 via suitable education means contained therein (not shown) and transported through the borehole 21 to the surface of the earth. The eduction means may be a perforated conduit disposed in the borehole extension 22. A similar perforated conduit may be disposed in borehole extension 18 to be used for injecting the heat-creating gas into the oil shale rubble. Other piping and related equipment needed for injecting the heat-creating gas and recovering of the shale oil at the surface are well known and have not been shown in order to simplify description.

The heat-creating gas having oxygen-containing gases may be preheated initially at the surface by suitable equipment (not shown) to a temperature sufficient to ignite spontaneously a portion of the hydrocarbonous materials in the oil shale rubble. The combustion of such material-s produces heat to decompose the kerogen and drive the resultant shale oil from the shale rubble. The heating moves progressively outwardly from the borehole extension to the limits of the rubble area to provide a uniform heat front to produce and drive the hydrocarbon products or shale oil from the shale into the borehole 21.

Once the combustion of the oil shale rubble is initiated, the temperature and velocity of the heat front can be regulated by adjusting the temperature of the heat-creating gas being introduced, or the volume of the gas and its oxygen content, to a degree best suited for maximum recovery of hydrocarbons from the oil shale.

In some instances, such as when the hydrocarbon content of the oil shale is relatively low, or for other reasons, combustible gases may also be injected into the oil shale rubble and therein ignited. Such combustible gases can provide the only fuel used for heating the shale or to supplement the hydrocarbons used as natural fuel in such shale. Some of the shale oil or its products may be recycled with the heat-creating gas if desired.

The combustible gases may be introduced through the borehole 17 along with the heat-creating gas. Various modes of using in situ combustion or heating may be used other than those described. This step of in situ heating by injecting various gases is well known to those skilled in the art.

A spherically shaped expanding heat front or wave is obtained by injecting the heat-creating gas through the lower borehole 17 as a result of the inherent tendency of the heat front or wave to pass not only outwardly from the borehole extension 18 but also upwardly. This is an advantage obtained by the more efficient in situ retorting of oil shale. Thus, less thermal-driving force is required for maximum production of shale oil.

Referring now to FIGURES 3 and 4, the priorly described embodiment of the present invention shown in FIGURES 1 and 2 will be described as adapted for producing a substantial horizontal portion of any horizon of the massive oil shale 13 and also to expanding the vertical extent of production vertically through successive horizons through the entire shale formation. Although a specific illustrative embodiment of such invention will be given, it is to be understood that various changes and alterations in and to the steps of the described process may be made as were described for the embodiment priorly described and shown in FIGURES l and 2. In this further embodiment there is provided a central borehole 31 extending from the surface of the earth downwardly into the oil shale 13. A plurality of borehole extensions 32 are directionally drilled from the central borehole 31 with the borehole extensions 32 having lower portions 33 diverging from the vertical toward the horizontal at the horizon in the oil shale 13 from which shale oil is to be recovered. The borehole extensions 32 are provided in the same manner as those in the priorly described embodiment shown in FIGURES l and 2. A plurality of peripheral boreholes 34 are provided about the central borehole 31 and extend from the earths surface downwardly into the oil shale 13. A multitude of radially disposed borehole extensions 36 are directionally drilled from each of the peripheral boreholes 34 with their lower portions 37 diverging from the vertical towards the horizontal. Preferably, the lower portions 37 are drilled on a course to intersect the lower portions 33 of the borehole extensions 32 of the central borehole 31. The lower portions 37 of the borehole extensions 36 of the peripheral boreholes 34 are disposed in a horizon different from that containing the borehole extensions 32 of the cen tral borehole 31. The borehole extensions 32 and 37 are spaced substantially Vertically at a distance permitting the intervening oil shale to be extensively fractured by conventional fracturing means applied from the borehole extensions to provide an area A" of oil shale rubble.

The formation adjacent to and intermediate borehole extensions 32 and 37 of the central and peripheral boreholes 31 and 34 is fractured by suitable fracturing means, such as hydraulic fracturing or explosives. The shale rubble area A produced by this fracturing step, which has great and uniform permeability, is surrounded by the unfractured massive oil shale which forms an effective retorting container.

After the rubble area A is formed, the next step is to heat the oil shale rubble to a retorting temperature sufficient to produce the desired shale oil. This heating is obtained by injecting a heat-creating gas through the borehole extensions 32 of the central borehole 31. This gas heats the oil shale rubble from adjacent such borehole extensions 32 progressively outwardly and upwardly to a temperature sufficient to release the hydrocarbons from the shale and to drive them into the borehole extensions 36 of the peripheral boreholes 34 from which the shale oil may be recovered. More specifically, with reference to the structure shown in FIGURE 4, injecting the heat-creating gas through the borehole extensions 3?. of the central borehole 31 provides an in situ combustion wave centrally disposed within the rubble area at its lower extremity. The created heat front assumes a circular configuration which provides the most efficient mode of retorting the oil shale rubble in area A with a minimum amount of heat-creating gas. Further, the natural forces predominantly drive the heat front upwardly and therefore the resultant heat front tends to move throughout the entire extent of the oil shale rubble area A. Thus, the possibility of by-passing unretorted portions of the oil shale rubble is negligible.

The retorted shale oil is recovered and removed from the borehole extensions 36 of the peripheral boreholes 34 by conventional eduction means as previously described. Other and further borehole extensions may be provided to both the central borehole 31 and the peripheral boreholes 34 at horizons closer to the surface of the earth. For example, in FIGURE 4, in chain-line, there are shown additional borehole extensions 38, respectively, for the central borehole 31 and other borehole extensions 39 for the peripheral boreholes 34. These borehole extensions 38 and 39 may be provided at the same or different times as the previously mentioned borehole extensions and structurally assume the same relationship. By repeating the previously described steps of fracturing and retorting of the oil shale adjacent such additional borehole extensions 38 and 39, the rubble area A may be progressively extended upwardly through the entire vertical extent of the massive oil shale. Thus, substantially all of the shale oil may be recovered therefrom.

The process described may be repeated over the horizontal portions of the shale formation desired to be retorted until the entire massive oil shale 13 is retorted and all the shale oil is recovered.

It will be readily appreciated from the foregoing description that herein is fully described a novel process well adapted to obtain maximum recovery of hydrocarbons from underground formations of oil shale and to obtain such hydrocarbons economically and that this process may be used as a means to produce vast quantities of hydrocarbons.

It will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations. This is contemplated by and is within the scope of the claims.

As many embodiments as possible may be made of the invention without departing from the scope thereof. It is to be understood that all matter herein set forth or shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. The process for recovering hydrocarbons from massive underground formations of oil shale comprising the steps of:

(a) providing a borehole extending from the earths surface downwardly into the oil shale, and directionally drilling from the borehole at spaced levels within the oil shale a plurality of first borehole extensions each having a lower portion diverging from the vertical toward the horizontal at different horizons in the shale from which oil is to be recovered;

(b) directionally drilling from a borehole which extends from the earths surface downwardly into the oil shale a plurality of second borehole extensions each having a lower portion diverging toward the horizontal from the vertical at different horizons within the oil shale, said second borehole extensions being positioned in spaced relationship to respective ones of said first borehole extensions by a distance permitting the intervening oil shale to be extensively fractured by conventional fracturing means;

(c) fracturing the formation adjacent to and intermediate at least one pair of the first and second borehole extensions;

(d) injecting a heat-creating gas through a first borehole extension located within the fractured oil shale of step (c) to heat the shale progressively outwardly to a temperature sufficient to drive the hydrocarbons in the shale to the respective second borehole extension;

(e) removing the recovered hydrocarbons from such second borehole extension; and

(f) repeating steps (c), (d) and (e) with other of the respective first and second borehole extensions.

2. The process for recovering hydrocarbons from massive underground formations of oil shale comprising the steps of:

(a) providing a first borehole extending from the earths surface downwardly into the oil shale, and directionally drilling a plurality of vertically spaced first borehole extensions each having a lower portion diverging from the vertical toward the horizontal at horizons within the shale from which oil is to be recovered;

(b) providing a second borehole adjacent to said first borehole which extends from the earths surface downwardly into the oil shale, and directionally drilling from said second borehole a plurality of vertically spaced second borehole extensions each having a lower portion diverging from the vertical toward the horizontal on a course to intersect the course of a respective one of said first borehole extensions, the lower portion of each second borehole extension being disposed in spaced relationship to the respective first borehole extension by a distance permitting the intervening oil shale to be extensively fractured by conventional fracturing means;

(0) fracturing the formation adjacent to at least the lowermost pair of respective first and second borehole extensions;

((1) injecting a heat-creating gas through the first borehole extension employed in step (c) into the fractured oil shale to heat the shale from adjacent said first borehole extension progressively outwardly to a temperature sufficient to drive the hydrocarbons in the shale to the respective second borehole extension;

(e) removing the recovered hydrocarbons from said respective second borehole extension; and

(f) subsequently repeating steps (c), (d) and (e) employing other of the respective first and second borehole extensions to recover additional hydrocarbons 2,788,956 4/1957 from other regions of the formation. 2,841,375 7/ 1958 References Cited by the Examiner 2,917,296 12/ 1959 UNITED STATES PATENTS 5 2,970,826 2/1961 1,422,204 7/1922 Hoover et a1. 16611 33017468 1/1962 1,816,260 7/1931 Lee 166-11 X 2,780,449 2/1957 Fisher et a1. 16611 X Pevere et a1. 16611 Salomonsson 16611 X Taderna 16611 Prentiss 16611 X Woodruff 16611 X Carr 16611 X BENJAMIN HERSH, Primary Examiner. 

1. THE PROCESS FOR RECOVERING HYDROCARBONS FROM MASSIVE UNDERGROUND FORMATIONS OF OIL SHALE COMPRISING THE STEPS OF: (A) PROVIDING A BOREHOLE EXTENDING FROM THE EARTH''S SURFACE DOWNWARDLY INTO THE OIL SHALE, AND DIRECTIONALLY DRILLING FROM THE BOREHOLE AT SPACED LEVELS WITHIN THE OIL SHALE A PLURALITY OF FIRST BOREHOLE EXTENSIONS EACH HAVING A LOWER PORTION DIVERGING FROM THE VERTICAL TOWARD THE HORIZONTAL AT DIFFERENT HORIZONS IN THE SHALE FROM WHICH OIL IS TO BE RECOVERED; (B) DIRECTIONALLY DRILLING FROM A BOREHOLE WHICH EXTENDS FROM THE EARTH''S SURFACE DOWNWARDLY INTO THE OIL SHALE A PLURALITY OF SECOND BOREHOLE EXTENSIONS EACH HAVING A LOWER PORTION DIVERGING TOWARD THE HORIZONTAL FROM THE VERTICAL AT DIFFERENT HORIZONS WITHIN THE OIL SHALE, SAID SECOND BOREHOLE EXTENSIONS BEING POSITIONED IN SPACED RELATIONSHIP TO RESPECTIVE ONES OF SAID FIRST BOREHOLE EXTENSIONS BY A DISTANCE PERMITTING THE INTERVENING OIL SHALE TO BE EXTENSIVELY FRACTURED BY CONVENTIONAL FRACTURING MEANS; 